Heterotrophs need to obtain energy from consuming autotrophs. Remember autotrophs turn light energy into chemical energy, which is stored as the sugar glucose. The process of releasing energy by breaking down glucose and other food in the presence of oxygen is called cellular respiration.

Cellular respiration is not exactly, but can be roughly viewed as, the reverse process as photosynthesis. Oxygen and glucose combine to breakdown and reassemble as carbon dioxide, water, and energy.

6O2 + C6H12O6 -> 6CO2 + 6H2O + ATP

oxygen + glucose -> carbon dioxide + water + energy

This process is simplified of course. There’s a whole confusing mess of glycolysis, ATP, and NADH which the average high school bio teacher would want you to know, but honestly it’s overkill (…it’s not even in our state standards…).

Now there are cases when cells don’t get enough oxygen. In this case, the cells produce nasty waste products that they remove from their body. Some microorganisms, such as yeast, produce alcohol in the absence of oxygen. This is called alcoholic fermentation. Other organisms, such as yourself, produce lactic acid in the absence of oxygen. Fittingly, this is called lactic acid fermentation. Fermentation is vital in our food system. Production of alcohol is quite the large business throughout the world; as is production of foods such as yogurt and pickles which utilize lactic acid.

One last point of overkill: the Krebs cycle and electron transport chain. Unless you go to college for Biology, you have no need for this… and honestly, I went to college for Biology and I don’t have a use for it, short of torturing students with it if I was an evil person.